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Tiny dark matter stars would harbour particles that act as one

9 December 2015

By Anna Nowogrodzki

(Image&colon; NIST)

“YOU will be assimilated.” In Star Trek, members of the strange and sinister race known as the Borg would utter this threat as if one. Their linked behaviour could be echoed in space if dark matter exists in a particular form – one that can build stars in which every particle is in the same state at the same time.

Dark matter is thought to account for 80 per cent of the matter in the universe, but hardly interacts with ordinary matter. This means it cannot be observed directly, so its constituents remain a mystery.

One theory suggests that it is made of hypothetical particles called axions. Unlike the protons, neutrons and electrons that make up ordinary matter, axions can share the same quantum state. They also attract each other via gravity, so they clump up.

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Those two properties mean that the clumps form a Bose-Einstein condensate (BEC) – a type of matter in which all the particles occupy the same quantum state, according to Chanda Prescod-Weinstein at the Massachusetts Institute of Technology and her colleagues. “They act like one super-atom together,” she says.

But the clumps are prone to falling apart. “The configuration the axions want to settle into is not one giant BEC,” she says. Instead, they end up as smaller clumps called Bose stars.

These would have formed when the universe was a mere 47,000 years old, and should survive to this day, she says. They would still be BECs and would end up relatively small – the size of the dwarf planet Ceres, and about 20 times as dense (Physical Review D, doi.org/9s5).

Although they do not shine, Bose stars could make it possible for us to observe dark matter, if they are orbiting a pulsar. Under the right conditions, the pulsar and the axions would interact to produce radiation we can pick up, says Prescod-Weinstein.

This article appeared in print under the headline “Dark matter could spawn Borg-like stars”